Add generic error message for flycheck

This is to catch situations where the is no file provided by the solc
compiler regarding the error.

For example in the following:

```
Error: Documented parameter "_longitude" not found in the parameter list of the function.
sikorka_basic_interface.sol:47:45: Error: Identifier not found or not unique.
    modifier need_pop(uint _latitude, uint, _longitude, string _answer)
    {
```

Author: LefterisJP

solidity-mode.el

@@ -437,7 +437,8 @@ Highlight the 1st result."
     "A Solidity syntax checker using the solc compiler"
     :command ("/usr/bin/solc" source-inplace)
     :error-patterns
-    ((error line-start (file-name) ":" line ":" column ":" " Error: " (message)))
+    ((error line-start (file-name) ":" line ":" column ":" " Error: " (message))
+     (warning line-start "Error: " (message)))
     :modes solidity-mode
     :predicate (lambda () (eq major-mode 'solidity-mode)))
   (add-to-list 'flycheck-checkers 'solidity-checker)